A heat exchanger comprises a plurality of parallel, spaced apart fins defining air flow passages. The fins are made of aluminum because of its excellent heat conductance and are designed to have the surface areas of the fins as large as possible in order to increase the heat radiation or cooling effect. The spaces between the fins are very narrow in order to pack more cooling area into as small a unit as possible. As a result, particularly when the exchanger is used for cooling, moisture from warm air passing through the unit condenses on the cold fin surfaces and, if those surfaces are hydrophobic, beads of water accumulate to block the narrow passages. The increased resistance to air flow reduces the efficiency of the heat exchanger. In order to prevent the accumulation of water between the fins, the surfaces of the fins are made hydrophilic so that a thin film of water coating the surface can slide off quickly, a phenomenon known in the industry as sheeting off.
Some of the methods known to make the surfaces more hydrophilic include coating the surface with an acrylic acid resin by itself or with a mixture of the resin with water glass (i.e., sodium silicate). In others, such as that taught in U.S. Pat. No. 4,181,773, the surface of an aluminum article is made water wettable by coating it with a continuous film of a water soluble basic polymer having colloidal alumina dispersed therein, curing the polymer to water insolubility, and then hydrolyzing the surface, only, of the cured polymer film. The basic polymer is exemplified by an organic acid salt of an epoxide resin containing free amine groups. In U.S. Pat. No. 4,503,907, a mixture of a water soluble acrylic resin and a water soluble amino resin is mixed with a synthetic silica and a surface active agent such as a polyoxyethylene glycol and the mixture is coated on the fin and baked. In U.S. Pat. No. 4,588,025, the fins are treated with a coating containing an alkali silicate and carbonyl-containing low molecular weight organic compound, such as an aldehyde, ester, or amide. Upon heating, the alkali silicate reacts with the carbonyl compound, which is converted into an organic carboxylate which is then incorporated into a three dimensional reticular polymer of the silicate. In U.S. Pat. No. 5,012,862, a coating of a polysulfonic acid is applied to a fin of an aluminum heat exchanger to render the surface hydrophilic.
The problem of providing a coating which will prevent the formation of white corrosion deposits, called white rust, on the aluminum fins is addressed in U.S. Pat. No. 4,421,789. This patent teaches that a water wettable corrosion resistant coating may be provided by depositing an ethylene/acrylic acid copolymer on an aluminum panel and then applying a silica coating on the first coating. The hydrophobic character of a styrene/acrylic acid copolymer may be modified, according to the published Japanese Patent Application No. 62-80494 [1987], by esterifying the acidic resin with a polyhydroxy alcohol.
There remains a need, however, for a simple water based system for applying a hydrophilic, corrosion resistant coating to a heat exchanger fin. It is an object of this invention, therefore, to provide a two-step--apply and dry--method for imparting corrosion resistance and water wettability to such fins.
It is another object of this invention to provide a metal-free coating composition for imparting corrosion resistance and water wettability to such fins.